Advanced Camera for Surveys Exposure Time Calculator: II. Baseline Tests for the Ramp Filter Modes.

Transcription

1 Instrument Science Report ACS Advanced Camera for Surveys Exposure Time Calculator: II. Baseline Tests for the Ramp Filter Modes. D. Van Orsow, F.R. Boffi, R. Bohlin, R.A. Shaw August 23, 2000 ABSTRACT The verification tests for the ACS Ramp Filter Exposure Time Calculator are presented. Our baseline suite of test cases includes one calculation for all filter modes with the same target, plus one subset for all kinds of targets through the same filter. 1. Introduction: This is the second in a series of three ISRs (the first was ISR ACS by Boffi et al.) where we summarize our testing of the Exposure Time Calculator (ETC) for ACS. Three interfaces to the ACS ETC are being developed; imaging, ramps and spectroscopic. Each interface consists of public and development versions for their supported modes. Once a complete set of testing has been carried out on the development version, the public version is updated and released. The public version web URL can be found at where simulator is either acs_ramp_etc, acs_img_etc or acs_spec_etc. 2. Our baseline of exposures for the Ramp ETC: In order to check the calculations for both CCD detectors and their ramp filters, we have created a baseline of exposures that spans the parameter space in two directions; we tested the ETC by keeping the spectral distribution fixed (flat spectrum) for all ramp filters, and also by keeping the filter fixed while varying the source spectral distributions. In Table 1, a subset of our test exposures are listed for the point source cases. The first six columns give the input parameters as detector, filter, source, normalization (in flux or magnitude), spectral distribution and exposure time. The filter column also contains the central wavelength of the filter bandpass used for the ramp filter. For example, filter fr914m#9000 indicates ramp filter fr914m where the wavelength of interest is 9000 Å. In the spectral 1

2 distribution column, flat indicates a constant flux distribution in erg sec -1 cm -2 Αng -1. P. Law indicates the power law spectrum of the form F( λ) = λ n (where the default n=- 1). BB indicates a black body spectrum of Temperture 10,000 Kelvin. The O5V, B5V, G5V, M5V and M5I entries are Kurucz model atmospheres. GD71 and G191B2B are standard stars. A one second CRSplit=1 exposure was run in all cases so that counts and counts sec -1 have the same numerical value. Default values for gain (2), sky (average), and boxsize (5x5 for WFC and 9x9 for HRC) are used throughout. Non-default boxsizes were also tested and the results agreed exactly with those predicted by ensquared energy tables for the various boxsizes. The last four columns of Table 1 give the output results: signal to noise ratio (SNR), brightest pixel, total source, and sky, where the last three columns are in electrons, i.e. counts. The STSDAS synthetic photometry package (synphot) was then run to compare with the ETC output. The ACS ETC and synphot agreed to within roundoff error in all cases. In addition to the subset of exposures in Table 1 we completely tested the remaining features of the ACS Ramp ETC. All remaining spectral distributions, including the Kurucz models, standard stars, non-stellar objects and the QSOs were tested successfully. Tests with various exposure times produced output which agreed with synphot. A user supplied spectrum of standard star GD71 was also tested. The exposure times computed by the ETC in order to reach our input SNR values were tested for the full baseline of exposures. This inverse type of test case is displayed for the fr388n filter in Table 1. The last WFC test case in Table 1 shows that the exposure time of one second is output when the same SNR of 3.1 is input to the ETC which we displayed in the first test case. 2

3 Table 1. Point Source Exposures Detector Filter Source Normal. Spectrum Exp time SNR Br. Pixel Source Sky wfc1 fr388n#3880 point 1e-15 flat wfc1 fr423n#4230 point 1e-15 flat wfc1 fr462n#4620 point 1e-15 flat wfc1 fr505n#5000 point 1e-15 flat wfc1 fr551n#5500 point 1e-15 flat wfc1 fr601n#6000 point 1e-15 flat wfc1 fr656n#6500 point 1e-15 flat wfc1 fr716n#7100 point 1e-15 flat wfc1 fr782n#7900 point 1e-15 flat wfc1 fr853n#8500 point 1e-15 flat wfc1 fr931n#9300 point 1e-15 flat wfc1 fr1016n#10000 point 1e-15 flat wfc1 fr459m#4620 point 1e-15 flat wfc1 fr647m#6470 point 1e-15 flat wfc1 fr914m#9000 point 1e-15 flat wfc1 fr388n#3880 point V=15 O5V wfc1 fr388n#3880 point V=15 B5V wfc1 fr388n#3880 point V=15 G5V wfc1 fr388n#3880 point V=15 M6V wfc1 fr388n#3880 point V=15 G5I wfc1 fr388n#3880 point NO GD wfc1 fr388n#3880 point NO G191B2B , wfc1 fr388n#3880 point V=20 G5V wfc1 fr388n#3880 point V=20 BB wfc1 fr388n#3880 point V=20 P. Law wfc1 fr388n#3880 point 1e-15 flat hrc fr388n#3880 point 1e-15 flat hrc fr505n#5050 point 1e-15 flat hrc fr656n#6560 point 1e-15 flat hrc fr459m#4590 point 1e-15 flat hrc fr388n#3880 point V=15 O5V

4 Detector Filter Source Normal. Spectrum Exp time SNR Br. Pixel Source Sky hrc fr388n#3880 point V=15 B5V hrc fr388n#3880 point V=15 G5V hrc fr388n#3880 point V=15 M6V hrc fr388n#3880 point V=15 G5I hrc fr388n#3880 point None GD hrc fr388n#3880 point None G191B2B , hrc fr388n#3880 point V=20 G5V hrc fr388n#3880 point V=20 BB hrc fr388n#3880 point V=20 P. Law In Table 2 extended sources are considered. The table columns are the same as in Table 1, except all boxsizes are 2x2 pixels. All SNR calculations for extended sources use the 2x2 boxsize, since the source size is assumed to be larger than the ACS resolution. A CRSplit=1 was also used throughout. The e. line spectrum is an emission line. Table 2. Extended Source Exposures Detector Filter Source Normal Spectrum Exp time SNR Br. Pixel Source Sky wfc1 fr388n#3880 extended V=22 flat wfc1 fr388n#3880 extended 1.e-15 e. line wfc1 fr459m#4590 extended V=22 flat wfc1 fr388n#3880 extended 1.e-15 flat hrc fr459m#4590 extended V=22 flat hrc fr388n#3880 extended 1.e-15 flat Checking exposures by hand: In the following, we present our hand calculations to check the ETC output for one extended continuum source and one extended emission line source. More examples are in Chapter 6 of the ACS Handbook along with the documentation for our equations. Extended Continuum Source: In Table 2, the ETC calculates SNR=0.5, the brightest pixel = 4.60, the source = 5.07 electrons and the sky = 2.18 for a 1000 second exposure of a flat extended source of V=22 mag. arcsec -2 using the Wide Field Channel and the fr388n ramp filter. The flux of a V=22 mag. arcsec -2 source is 6x10-18, the sensitivity of the filter is x10 15 and the 4

5 FWHM= The calculations are performed over a 2x2 pixel box. The total counts from the extended source is approximately x10 15 *6x10 18 *4*( * )*87.558*1000=5.2 electrons which is 2% from the more accurate value of 5.1 calculated by the ETC integrating over the passband. This integration over the Å passband is responsible for most of the 2% error. An error greater than 10% would be cause for concern. The dark rate is 1000*2.78x10-4 = 11, while the brightest pixel is 0.25 * ( ) = 4.6 which agree with the ETC. Extended Emission Line Source: In Table 2, the ETC calculates what number of counts are achieved for a 1 sec exposure for an emission line at 3880 Å of specific intensity 1.x10-15 erg sec -1 cm -2 arcsec -2, using the Wide Field Channel and the fr388n ramp filter, in the case of an extended source (CRSplit=1). The sensitivity is calculated as S 3880 = 2.27x10 12 * T 3880 * 3880=1x10 15, where 2.27x10 12 is a constant, T 3880 is the system throughput at the wavelength of 3880 Å, and 3880 is the wavelength. The counts per pixel in the line are simply given by the product of specific intensity, sensitivity and WFC pixel size squared ( ). The value that we obtain is , that multiplied by 4 pixels (which is the number of pixels used by the ETC for the calculation of the S/N) gives 9.8x10-3 which is 1% from the ETC value of 9.9x10-3. Since the emission line source calculation does not calculate over an 87.5 Å passband like the previous extended source calculation, it avoids that error and should be within ~1% of the ETC value. The detector dark is equal to 2.78x10-3 *4=0.011, like the ETC. The average sky value is calculated as one half the continuum high sky background of 5x10-18 erg sec -1 cm -2 Å -1 arcsec -2, from Figure 6.1: 0.5 * 5x10-18 * 1 x10 15 * (0.049) 2 * 4 * * 1 = 0.002, which is also the value calculated by the ETC. The brightest pixel contains 0.25 of the total source, sky, and dark rate: 0.25 * ( ) = , which agrees with the ETC value of Finally, we apply the usual signal-tonoise ratio equation and obtain SNR=9.9x10-3 /(9.9x ) 0.5 = 0.001, which is what the ETC calculates. 4. References Boffi F.R., Bohlin R.C. & Shaw R.A. 2000, Advanced Camera for Surveys Exposure Time Calculator: I. Baseline Test for Broadband Imaging Modes, ISR ACS (Baltimore: STScI). Jedrzejewski, R., et al. 2000, ACS Instrument Handbook, version 1.0, (Baltimore: STScI). 5